LEDs are built on a substrate and are doped with impurities to create a p-n junction. A current flows from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Electrons and holes flow into the p-n junction from electrodes with different voltages. If an electron combines with a hole, it falls into a lower energy level and releases energy in the form of a photon. The wavelength of the light emitted by the LED and the color of the light may depend on the band gap energy of the materials forming the p-n junction.
LED devices are typically formed by initially forming a stack of layers in which one or more layers are p-type semiconductors and one or more layers are n-type semiconductors, such that a p/n junction forms within the layer stack. The stack of layers may be formed on a planar insulating substrate in some cases. The insulating substrate in an LED may be, for example, sapphire. Vertical LED structures include a p-contact (electrode) on one side of the stack of layers while an n-contact is formed on the other side of the stack of layers. Lateral LED structures include a p-contact and n-contact on the same side of a substrate (or same side of a stack of layers).
FIG. 1 illustrates a cross-section of a known lateral LED structure 100 formed using GaN as the semiconductor material. LED 100 includes a p-contact 102 that is used to contact p-GaN layer 104 on its exposed outer surface. An InGaN quantum well structure 106 is sandwiched between p-layer 104 and an n-GaN layer 108. An exposed region of the n-GaN layer forms a mesa 122 that is recessed with respect to the surface of the p-GaN layer. LED 100 also includes an n-contact 110 that is formed on the same side of substrate 112 as the p-contact 102 on an exposed surface of n-GaN layer 108. A buffer layer 114 is also formed to help match the substrate to the n-GaN layer 108.
The stepped structure of LED 100 in which a portion of the inner layer 108 has an exposed outer surface facing the same direction as the outer surface of p-layer 104 allows non-buried planar contacts to be formed on the same side of the layer stack 104-114, in which the contacts 102, 110 are displaced laterally from one another along the x-direction as shown. Accordingly, although the current 116 travels across the p/n junction of LED 100 in z-direction normal to the P/N junction (a vertical direction for the LED orientation shown in FIG. 1), the current must travel in a horizontal fashion (x-direction) generally parallel to the p/n junction between n-contact 110 and the region of the p/n junction 118, which is formed between layers 104 and 108. In the vicinity of contact 110 the current changes direction between a predominantly horizontal flow and a more vertical flow that exists immediately under at least the edge 120 of contact 110.
Such a lateral LED structure may therefore suffer from current crowding near the contacts, especially the re-contact, which degrades the LED performance. Current always takes a path of least resistance, which for the case of lateral LED structure 100 may be across or near the edge 120 of the contact 110. Thus, the current may not spread under the entire contact length L. In one instance, the voltage may be highest at the contact edge and drop exponentially with distance from the contact. Such non-uniform current spreading near LED contacts results in localized joule heating and light emission. This may cause color binning, early saturation of light intensity, and a short LED device lifetime.
Accordingly, it is desirable to provide improvements over present day LEDs.